Edge termination for silicon power devices

ABSTRACT

A silicon semiconductor die comprises a heavily doped silicon substrate and an upper layer comprising doped silicon of a first conduction type disposed on the substrate. The upper layer comprises a well region of a second, opposite conduction type adjacent an edge termination zone that comprises a layer of a material having a higher critical electric field than silicon. Both the well region and adjacent edge termination zone are disposed at an upper surface of the upper layer, and an oxide layer overlies the upper layer and the edge termination zone. A process for forming a silicon die having improved edge termination. The process comprises forming an upper layer comprising doped silicon of a first conduction type on a heavily doped silicon substrate, and forming an edge termination zone that comprises a layer of a material having a higher critical electric field than silicon at an upper surface of the upper layer. A well region of a second, opposite conduction type is formed at the upper surface of the upper layer adjacent the edge termination zone, and an oxide layer is formed over the upper layer and edge termination zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 09/344,868, filed Jun. 28, 1999 now U.S. Pat. No. 6,242,784.

FIELD OF THE INVENTION

The present invention relates to silicon power semiconductor devices and, more particularly, to a silicon semiconductor die having an efficient and reliable edge termination zone.

BACKGROUND OF THE INVENTION

PN junctions within semiconductor devices are not infinite, terminating at the edge zones of a die. This edge effect limits the device breakdown voltage below the ideal value, V_(brpp), that is set by the infinite parallel plane junction. Care must be taken to ensure proper and efficient termination of the junction at the edge of the die; if the junction is poorly terminated, the device breakdown voltage can be as low as 10-20% of the ideal case. Such severe degradation in breakdown voltage can seriously compromise device design and lead to reduced current rating as well. In addition, an inefficient edge termination makes a device unstable and unreliable if the device is operated in a harsh environment or over a long period of time.

Various edge termination techniques have been developed, including, for example, field plate (FP), described in F. Conti and M. Conti, “Surface breakdown in silicon planar diodes equipped with field plate,” Solid State Electronics, Vol. 15, pp 93-105, the disclosure of which is incorporated herein by reference. Another edge termination approach is field limiting rings (FLR), described in Kao and Wolley, “High voltage planar p-n junctions,” Proc. IEEE, 1965, Vol. 55, pp 1409-1414, the disclosure of which is incorporated herein by reference. Further edge termination structures utilized variable lateral doping concentration (VLD), described in R. Stengl et al., “Variation of lateral doping as a field terminator for high-voltage power devices, IEEE Trans. Electron Devices, 1986, Vol. ED-33, No. 3, pp 426-428, and junction termination extension (JTE), described in V.A.K. Temple, “Junction termination extension, a new technique for increasing avalanche breakdown voltage and controlling surface electric field in p-n junction,” IEEE International Electron Devices Meeting Digest, 1977 Abstract 20.4, pp 423-426, the disclosures of which are incorporated herein by reference.

The purpose of all these various techniques is to reduce electron-hole avalanche generation by lowering the peak electric field strength along the semiconductor surface and thereby shifting the avalanche breakdown location into the bulk of the device. To achieve this goal, the width of the edge termination zone (L_(edge)) has to be several times higher than the depletion width (W_(pp)) of the parallel-plane portion of the PN junction. For example, if L_(edge)=2.98W_(pp), 98.7% of V_(brpp) can be achieved when an “ideal edge termination,” as described in Drabe and Sittig, “Theoretical investigation of plane junction termination,” Solid State Electronics, 1996, Vol. 3, No. 3, pp 323-328, the disclosure of which is incorporated herein by reference, is used. In practice, a longer L_(edge) than the theoretical value should be used to guarantee device reliability. However, it is very important to point out that, even with very efficient edge termination, electron-hole impact generation at a rate of about 1×10¹⁸ pairs/cm³.s, still exists along the semiconductor surface.

SUMMARY OF THE INVENTION

A silicon semiconductor die of the present invention comprises a heavily doped silicon substrate and an upper layer comprising doped silicon of a first conduction type disposed on the substrate. The upper layer comprises a well region of a second, opposite conduction type adjacent an edge termination zone that comprises a layer of a material having a higher critical electric field than silicon. Both the well region and adjacent edge termination zone are disposed at an upper surface of the upper layer, and an oxide layer overlies the upper layer and the edge termination zone.

Further in accordance with the present invention is a process for forming a silicon die having improved edge termination. The process comprises forming an upper layer comprising doped silicon of a first conduction type on a heavily doped silicon substrate, and forming an edge termination zone that comprises a layer of a material having a higher critical electric field than silicon at an upper surface of the upper layer. A well region of a second, opposite conduction type is formed at the upper surface of the upper layer adjacent the edge termination zone, and an oxide layer is formed over the upper layer and edge termination zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the formation of an edge zone in an silicon die by implantation and diffusion.

FIGS. 2-4 schematically depict the formation of a silicon die having a silicon carbide edge zone of a selected thickness.

FIGS. 5 and 6 illustrate the leakage current and electron-hole impact ionization generation contours at the onset of edge breakdown for a prior art die having a field plate.

FIGS. 7 and 8 depict the leakage current and electron-hole impact ionization generation contours at the onset of edge breakdown for a die of the present invention that includes a field plate.

FIGS. 9 and 10 illustrate the leakage current and electron-hole impact ionization generation contours at the onset of edge breakdown for a second prior art die having a field plate, a thin oxide layer, and a deep P-well.

FIGS. 11 and 12 depict the leakage current and electron-hole impact ionization generation contours at the onset of edge breakdown for a second embodiment of the present invention, wherein the die does not include a field plate.

FIG. 13 is a plot comparing electron-hole avalanche generation rates for silicon dies of the prior art and the present invention.

FIGS. 14A and 14B compare surface depletion layer boundaries in dies of the prior art and the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides more efficient and reliable edge termination for silicon power semiconductor devices compared with currently known techniques. In accordance with the invention, the silicon within an edge zone of a silicon die is replaced with a material having a higher critical electric field (E_(crit)), which is the maximum electric field under breakdown condition, and a lower impact ionization generation rate, which is the number of electron-hole pairs generated by an electron or a hole per unit distance traveled.

Replacement of silicon with a suitable material for this purpose can be accomplished in several ways, including, for example, implantation or deposition, or by heteroepitaxial growth, as described, for example, in Madapura et al., “Heteroepitaxial Growth of SiC on Si (100) and (111) by Chemical Vapor Deposition Using Trimethylsilane,” Journal of the Electrochemical Society, 1999, Vol. 46, No. 3, pp 1197-1202, the disclosure of which is incorporated herein by reference. SiC , because of its high E_(crit) (˜12 times higher than Si) and compatible thermal oxidation process with silicon, is a useful replacement material for silicon in a die edge zone. Possible processes to produce the SiC material edge to a controlled depth in a silicon substrate are illustrated in FIGS. 1-4.

FIG. 1 illustrates the implantation of carbon, C, into the edge zone 101 of a die 100, shown as an N-epitaxial layer, using an oxide mask 102. A high temperature process, such as laser-promoted local annealing, can be used to activate and diffuse the C implant, resulting in the edge surface layer 103 of silicon die 100 being converted to SiC.

FIGS. 2-4 illustrate another possible process to make a SiC die edge zone. First, a recessed edge zone 201 is etched by either a dry or a wet etch procedure to a specified depth in a silicon die 200, represented as an N-epitaxial layer, using an oxide mask 202 to prevent silicon removal from the active region 203, as shown in FIG. 2. A SiC layer 204, which is of the same conduction type as the epitaxial layer, is formed on recessed edge zone 201 using heteroepitaxial growth or deposition techniques, as shown in FIG. 3. Oxide mask 202 is then removed, and chemical-mechanical polishing (CMP) is performed to produce a fully planarized die 205 that includes SiC edge zone 204 a having a selected thickness, as demonstrated in FIG. 4.

Computer simulations have been performed to verify the concept for a wide range of breakdown voltages, from 600V to 30V. Simulated leakage current and electron-hole impact ionization generation contours, 511 and 508, respectively, at the onset of edge breakdown for a prior art die 500 with field plate edge termination are shown in FIGS. 5 and 6, respectively. Die 500 includes a substrate 501 bearing an N-epitaxial layer 502, in which is implanted a P-well 503. On the surface 504 of epitaxial layer 502 is deposited an oxide layer 505 and, in contact with P-well 503, a front metal layer 506 that is further in contact with a field plate 507. A back metal layer (not shown) is formed on the bottom of substrate 501.

As shown in FIG. 6, the highest electron-hole generation site 508 is located close to the intersection of the PN junction 509 between P-well 503 and N-epitaxial layer 502 and silicon upper surface 504. This is due to the termination of PN junction 509 to form a planar diffused junction having a finite curvature, which causes electric field crowding near upper surface 504 and leads to large impact ionization values at the die edges. Consequently, the breakdown occurs at junction termination edge 508 rather than in the parallel plane portion 510.

The breakdown characteristics of a die 700 of the present invention having a field plate edge and a SiC edge zone formed by C implantation and diffusion have also been simulated. Die 700, schematically depicted in FIGS. 7 and 8, includes a substrate 701 bearing an N-epitaxial layer 702, in which is implanted a P-well 703. At the surface 704 of epitaxial layer 702 is formed a SiC edge zone 705. An oxide layer 706 is formed on SiC edge zone 705, and a front metal layer 707 interconnects P-well 703 with a field plate 708. A back metal layer (not shown) is formed on the bottom of substrate 701.

FIGS. 7 and 8 illustrate the simulated avalanche leakage current and impact ionization contours, 709 and 710, respectively, for die 700. The breakdown location 710 is completely screened from the oxide layer 706 by SiC edge zone 705, and there is very little electron-hole generation along upper surface 704. The breakdown voltage for die 700 is higher than that observed for die 500.

In order to reduce the electron-hole avalanche generation rate along the Si/oxide interface and improve device reliability, a prior art die 900 makes use of a deeper PN junction and thinner oxide to lower the curvature effect. Die 900 with field plate edge termination includes a substrate 901 bearing an N-epitaxial layer 902, in which is implanted a deep P-well 903. On the surface 904 of epitaxial layer 902 is deposited a thin oxide layer 905 and, in contact with P-well 903, a front metal layer 906 that is further in contact with a field plate 907. A back metal layer (not shown) is formed on the bottom of substrate 901.

FIGS. 9 and 10 illustrate the simulated avalanche leakage current and impact ionization contours, 908 and 911, respectively, for prior art die 900. By properly choosing the depth of PN junction 909 and the thickness of oxide layer 905, the breakdown location is moved to the parallel plane portion 910 of PN junction 909. As a result, the device reliability of die 900 can be substantially improved. However, although the avalanche breakdown location is shifted into the bulk silicon, there still exists a certain level of electron-hole generation along the interface 904 between epitaxial layer 902 and oxide layer 905. The simulation gives an impact ionization generation rate at breakdown location 911 of about 1×10²¹ pairs/cm³.s and a generation rate at the same voltage of about 1×10¹⁸ pairs/cm³.s at surface 904.

The present invention provides further improvement, without the need for changing junction depth and oxide thickness, over the results obtained with prior art die 900. Die 1100, schematically depicted in FIGS. 11 and 12, includes a substrate 1101 bearing an N-epitaxial layer 1102, in which is implanted a P-well 1103. At the surface 1104 of epitaxial layer 1102 is formed a SiC edge zone 1105 that extends into N-epitaxial layer 1102 to a depth below that of P-well 1103. An oxide layer 1106 is formed on SiC edge zone 1105, and a front metal layer 1107 interconnects P-well 1103. Unlike previously described dies, die 1100 includes no field plate. A back metal layer (not shown) is formed on the bottom of substrate 1101.

By making the SiC edge layer deeper than the planar PN junction, edge termination with the ideal breakdown voltage can be achieved. Furthermore, the field plate can be omitted without degrading the device breakdown characteristics. FIGS. 11 and 12 illustrate the simulated avalanche leakage current and impact ionization contours, 1110 and 1111, respectively, for die 1100 of the present invention. The breakdown location 1108 is optimally situated at P-N junction parallel plane portion 1109. In addition, the electron-hole generation at upper surface 1104 is extremely low.

FIG. 13, a plot of impact ionization along the Si/SiO₂ interface versus distance from the P-N junction at the interface, depicts the surface carrier generation characteristics of the field plate-containing prior art dies 500 (cf. FIGS. 5, 6) and 900 (cf. FIGS. 9, 10) shown in FIGS. 5 and 9, along with die 1100 (cf. FIGS. 11, 12) of the present invention. The SiC edge termination included in die 1100 lowers the electron-hole avalanche generation rate more than 20 orders of magnitude compared with prior art die 500, and more than 16 orders of magnitude compared with prior art die 900. Furthermore, the breakdown voltage of die 1100 is desirably increased as a result of the thicker net epitaxial layer, which is defined by the distance between the parallel plane portion 1109 of the PN junction and highly doped substrate 1101.

Another improvement provided by the present invention is a reduction in edge termination area, which is controlled by the width of the surface depletion layer. Edge termination in accordance with the present invention does not change the curvature of the edge planar junction and the equal-potential contour distributions. Therefore the width of the surface depletion layer, which is less than the depletion width of the parallel plane portion, does change. According to the analysis described in the previously mentioned paper of Drabe and Sittig, the area of edge termination in die 1100 is expected to be about half that of the theoretical “ideal” Si edge termination.

In the absence of any termination structures, the width of the edge zone containing material with a higher critical electrical field than silicon can be chosen to be equal to the width of the surface depletion layer of the edge planar junction. To verify this, the width of the SiC edge zone 1105 in die 1100 (cf. FIG. 12) is reduced to correspond to the surface depletion layer boundary 1112 of the PN junction 1109. The simulated breakdown characteristic does not change, and the breakdown voltage also remains the same. The depletion layer boundary 1112 of die 1100 at the onset of avalanche breakdown is shown in FIG. 14A. The depletion layer boundary 912 of field plate-containing die 900 (cf. FIG. 10) is depicted in FIG. 14B. The width of the depletion layer in prior art die 900 is at least two times greater than that of die 1100 of the present invention.

In addition to the described field plate (FP), the edge termination of the present invention can be advantageously applied in semiconductor dies that include other edge terminating features such as, for example, field limiting rings (FLR), variable lateral doping concentration (VLD), and junction termination extension (JTE).

The present invention has been described in detail for the purpose of illustration, but it is understood that such detail is strictly for that purpose, and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the following claims. 

What is claimed:
 1. A process for forming a silicon die having improved edge termination, said process comprising: forming an upper layer comprising doped silicon of a first conduction type on a heavily doped silicon substrate; forming an edge termination zone at an upper surface of said upper layer, said edge termination zone comprising a layer of a material having a higher critical electric field than silicon; forming a well region of a second, opposite conduction type in said upper layer adjacent said edge termination zone; and forming an oxide layer over said upper layer and said edge termination zone.
 2. The process of claim 1 wherein said upper layer is an epitaxial layer.
 3. The process of claim 1 wherein said first conduction type is N and said second conduction type is P.
 4. The process of claim 1 wherein said first conduction type is P and said second conduction type is N.
 5. The process of claim 1 wherein said edge termination zone has a selected thickness.
 6. The process of claim 1 wherein said edge termination zone comprises a layer of silicon carbide.
 7. The process of claim 6 wherein said forming said layer of silicon carbide comprises implanting, activating, and diffusing carbon into said upper silicon layer.
 8. The process of claim 6 wherein said forming said edge termination zone comprises etching said upper surface of said upper layer prior to forming said layer of silicon carbide, thereby providing an edge termination zone recessed in said upper layer.
 9. The process of claim 8 wherein said edge termination zone recessed in said upper layer extends into said upper layer to a depth exceeding the depth of the adjacent well region. 